As electric devices are being recently increasingly used, harmonics current generated at the load sides are flown into an electric power system via a neutral line, to thereby cause lowering of the output, burning of coils, lowering of a power factor, and a power loss of an electric generator or transformer. Thus, a device for reducing harmonics current should be necessarily installed in the electric power lines.
Most of the conventional harmonics reduction methods are performed by employing reactors installed between a neutral line of an electric power system and that of each load side, in order to reduce harmonics waves. However, the reactors may be overheated by loads such as direct-current equipment, high-speed electric railways, mercury discharging lamps, star-connection wired transformers which generate much harmonics current and thus wires are frequently disconnected or cut. And the harmonics reduction effect by the reactors may be badly fluctuated and the wires would be cut owing to the overdue earth fault current. Accordingly, the neutral line may be disconnected or cut, to accordingly cause problems that the voltage at the load side is not balanced, and the maximum two-times voltage is applied to the load.
To prevent these problems, a device for reducing zero-phase harmonics components was proposed in a Korean patent registration No. 364513 (Laid-open patent publication No. 2001-0083809 published on 3 Sep. 2001 entitled “Device for reducing harmonics in a power line”), in which neutral line reactors are not used, and a zigzag transformer is installed between an electric power system and a load system. FIG. 1 is a circuitry diagram showing the harmonics reduction device. The harmonics reduction device includes: an electric power system 15 having three-phase poly-wire power lines in order to supply alternating-current (AC) power voltage to a load 10; a zigzag transformer 30 in which windings connected with three-phase lines in the electric power system 15 are wound through first and second legs 21 and 22, second and third legs 22 and 23, and third and first legs 23 and 21 of a core 20 having three legs 21, 22 and 23, respectively, while varying direction of each magnetic flux, and an intermediate point of each winding is commonly connected with a neutral line of the electric power system 15 and that of the load 10; and a line reactor 40 which is connected along each phase line between the electric power system 15 and the zigzag transformer 30. The harmonics reduction device does not use a reactor along the neutral line, and thus prevents the neutral line from being disconnected or cut. Also, the harmonics reduction device reduces zero-phase harmonics due to offset of a magnetic flux via the zigzag transformer 30, and thus prevents damage of the electric power system 15 due to zero-phase impedance.